1. Field of the Invention
The present invention relates to an apparatus and method for diagnosing an anomaly of a gas sensor.
2. Description of the Related Art
Conventionally, in order to reduce the amount of CO, NOX and HC contained in exhaust gas of an internal combustion engine, a technique has been employed of detecting the concentration of a specific component (e.g., oxygen concentration) within the exhaust gas, and controlling the air-fuel ratio of a gas mixture supplied to the internal combustion engine in accordance with the detected concentration. For example, a lamda sensor in which a solid electrolyte is used as a sensor element and whose output assumes one of two values (representing rich and lean states, respectively) or a full range air-fuel ratio sensor is used as a gas sensor for detecting the concentration of the specific component within the exhaust gas.
In general, these gas sensors are configured as follows. A plate-shaped or bar-shaped sensor element is held within a cylindrical tubular metallic shell such that a gas detection section at the front end of the sensor element is exposed; and the exposed portion is covered with a protective cover. Electrodes for outputting a signal representing the concentration of a gas are provided on the surface of a rear end portion of the sensor element, and connection terminals of lead wires for leading the signal outside the gas sensor are in contact with the electrodes (refer to Patent Documents 1 and 2).    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2006-300923    [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2006-308328    [Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 2006-343317    [Patent Document 4] Japanese Patent Application Laid-Open (kokai) No. H10-48180
As described above, the signal which represents the concentration of the gas is output outside the gas sensor via contact interfaces between the electrodes of the sensor element and the connection terminals of the lead wires. Therefore, if either of the contact interfaces has a contact resistance equal to or greater than a prescribed level, the contact resistance adversely affects the output of the gas sensor, whereby accuracy in measurement of the gas concentration is reduced.